The present disclosure relates to a disc drive and a tracking servo pull-in method thereof, and more particularly to a disc drive and a tracking servo pull-in method thereof which are very suitable for a case of recording and reproducing an optical disc recording medium in which position guiders are not formed in advance at a layer position where marks are to be recorded and recording parts and non-recording parts are mixed at the layer position.
As optical disc recording media (optical discs) for recording and reproducing signals using light emission, for example, a CD (Compact Disc), a DVD (Digital Versatile Disc), a BD (Blu-ray Disc: registered trademark), and the like have become widespread.
The present applicant has proposed a so-called bulk recording type optical recording medium as disclosed in Japanese Unexamined Patent Application Publication No. 2008-135144 or Japanese Unexamined Patent Application Publication No. 2008-176902 with regard to optical disc recording media which lead the next generation of optical disc recording media which are widespread at present such as CDs, DVDs, BDs, and the like.
Here, the bulk recording is a technique in which, for example, as shown in FIG. 25, laser light emission is performed for an optical recording medium (a bulk type recording medium 100) having at least a cover layer 101 and a bulk layer (recording layer) 102 while sequentially changing focal positions and thus multi-layer recording is performed inside the bulk layer 102, thereby achieving a large recording capacity.
For such bulk recording, a recording technique called a micro hologram type is disclosed in Japanese Unexamined Patent Application Publication No. 2008-135144.
In the micro hologram type, a so-called hologram recording material is used as a recording material of the bulk layer 102. As the hologram recording material, for example, light cured photopolymer or the like is widely known.
Micro hologram types are largely classified into a positive micro hologram type and a negative micro hologram type.
The positive micro hologram type is a method in which two light beams (light beam A and light beam B) opposite to each other are collected at the same position so as to form fine interference fringes (holograms), which are used as recording marks.
In addition, the negative micro hologram type is a method in which, in contrast to the positive micro hologram type, interference fringes which are formed in advance are erased by laser light emission, and the erased portions are used as recording marks. Specifically, in the negative micro hologram type, an initialization process for forming interference fringes on the bulk layer 102 in advance is performed before a recording operation is performed. The initialization process is performed by irradiating the bulk layer 102 with light beams C and D by parallel light to be opposite to each other, and forming interference fringes on the overall bulk layer 102. Further, the interference fringes are formed in advance through the above-described initialization process, and then information is recorded by forming erasure marks. Specifically, in a state of focusing on an arbitrary layer position, information is recorded using the erasure marks by performing laser light emission according to information to be recorded.
Further, the present applicant has proposed a recording method of forming voids (vacancies) as recording marks, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2008-176902, as a method of the bulk recording different from the micro hologram type.
The void recording method is a method in which laser light emission is performed for the bulk layer 102 made of a recording material such as, for example, light cured photopolymer at relatively high power, thereby recording voids inside the bulk layer 102. As disclosed in Japanese Unexamined Patent Application Publication No. 2008-176902, the vacancy portions formed in this way have a refractive index different from other portions in the bulk layer 102, and thus reflectance of light at the interfaces can be heightened. Therefore, the vacancy portions function as recording marks, and thereby information recording is realized by the formation of the vacancy marks.
Since the void recording type does not form holograms, recording may be completed through light emission from one side. In other words, it is not necessary to collect two light beams at the same position and form recording marks unlike the positive micro hologram type.
Upon comparison with the negative micro hologram type, there is an advantage in that the initialization process is not necessary.
In addition, although an example where when the void recording is performed, pre-cure light is applied before the recording is described in Japanese Unexamined Patent Application Publication No. 2008-176902, the void recording can be performed even if the application of the pre-cure light is omitted.
However, the bulk recording type (hereinafter, simply referred to as a bulk type) optical recording medium where the above-described variety of recording methods are proposed does not have an explicit multi-layer structure in the meaning that the recording layer (bulk layer) of the bulk type optical recording medium is formed by, for example, a plurality of recording films (reflection films). That is to say, the bulk layer 102 is not provided with a plurality of recording films and position guiders for each recording film which a typical multi-layer disc has.
Therefore, in a state of the structure of the bulk type recording medium 100 shown in FIG. 25 described above, a focus servo or a tracking servo may not be performed during the recording where the marks are not formed.
For this reason, in practice, the bulk type recording medium 100 is provided with a reflection surface (reference face Ref) which has guide grooves as shown in FIG. 26 and is used as a reference.
Specifically, guide grooves (position guider) are formed at the lower surface side of the cover layer 101 by, for example, formation of pits or grooves and a selective reflection layer 103 is formed thereon. In addition, the bulk layer 102 is laminated on the lower layer side of the cover layer 101 where the selective reflection layer 103 is formed in this way, via an adhesive material such as, for example, a UV cured resin as an intermediate layer 104 in the figure.
After the above-described medium structure is formed, as shown in FIG. 27, the bulk type recording medium 100 is irradiated with servo laser light as laser light for position control independently from laser light for recording marks (hereinafter, recording laser light).
As shown in the figure, the recording laser light and the servo laser light are applied to the bulk type recording medium 100 via a common objective lens.
At this time, if the servo laser light reaches the bulk layer 102, there is concern that the servo laser light may have an adverse effect on the mark recording in the bulk layer 102. For this reason, in the bulk recording type in the related art, laser light having a wavelength band different from that of the recording laser light is used as the servo laser light, and, as a reflection layer formed on the guide groove formation surface (reference face Ref), the selective reflection layer 103 having wavelength selectivity of reflecting servo laser light and transmitting recording laser light therethrough is provided.
Based on the above-described premise, an operation of when marks are recorded on the bulk type recording medium 100 will be described with reference to FIG. 27.
First, when multi-layer recording is performed for the bulk layer 102 which does not have guide grooves or a reflection layer, a layer position where the marks are recorded in the bulk layer 102 in the depth direction is set in advance. In the figure, as a layer position where marks are formed (mark forming layer position: also referred to as an information recording layer position) in the bulk layer 102, a case is exemplified where a total of five information recording layer positions L of a first information recording layer L1 to a fifth information recording layer position L5 are set. As shown, the first information recording layer position L1 is set at a position separated from the selective reflection layer 103 (reference face Ref) on which the guide grooves are formed, in the focus direction (depth direction) by a first offset of-L1. In addition, the second information recording layer position L2, the third information recording layer position L3, the fourth information recording layer position L4, and the fifth information recording layer position L5 are respectively set at positions separated from the reference face Ref by a second offset of-L2, a third offset of-L3, a fourth offset of-L4, and a fifth offset of-L5.
Here, during recording where the marks are not formed, a focus servo or a tracking servo may not be performed for each layer position L in the bulk layer 102 based on reflection light of the recording laser light. Therefore, a focus servo control and a tracking servo control of the objective lens during the recording are performed such that a spot position of the servo laser light tracks the guide grooves on the reference face Ref based on reflection light of the servo laser light as position control light.
However, it is necessary for the recording laser light to reach the bulk layer 102 formed at the lower layer side of the selective reflection layer 103 for the mark recording. For this reason, an optical system in this case is provided with a focus mechanism for independently adjusting a focus position of the recording laser light separately from the focus mechanism of the objective lens.
Here, FIG. 28 shows an internal configuration example of a recording apparatus of the bulk type recording medium 100 including a mechanism for independently adjusting a focus position of the recording laser light.
In FIG. 28, a first laser diode 111 denoted by LD1 is a light source of recording laser light, and a second laser diode 119 denoted by LD2 is a light source of servo laser light. As can be seen from the above description, the first laser diode 111 and the second laser diode 119 are configured to emit laser light having wavelength bands different from each other.
As shown in the figure, recording laser light emitted from the first laser diode 111 is incident to a focus mechanism including a fixed lens 113, a movable lens 114, and a lens driving portion 115, via a collimation lens 112. When the movable lens 114 is driven in a direction parallel to the optical axis of the recording laser light by the lens driving portion 115, a collimation state (divergence, parallel, and convergence) of the recording laser light incident to an objective lens 117 in the figure varies, and thus a focus position of the recording laser light can be adjusted independently from variations of a focus position due to driving of the objective lens 117.
In addition, in this meaning, the focus mechanism is also indicated as a recording light focus mechanism.
The recording laser light is incident via the recording light focus mechanism to a dichroic mirror (dichroic prism) 116 which is configured to transmit light having the same wavelength band as the recording laser light therethrough and reflect light other than that.
As shown in the figure, the recording laser light passing through the dichroic mirror 116 is applied to the bulk type recording medium 100 via the objective lens 117. The objective lens 117 is maintained so as to be displaced in a focus direction and a tracking direction by a biaxial actuator 118.
In addition, servo laser light emitted from the second laser diode 119 is transmitted through a beam splitter 121 via a collimation lens 120 and is incident to the above-described dichroic mirror 116. The servo laser light is reflected by the dichroic mirror 116 and is incident to the objective lens 117 such that the optical axis thereof matches the optical axis of the recording laser light which is transmitted through the dichroic mirror 116.
The servo laser light which is incident to the objective lens 117 focuses on the selective reflection layer 103 (reference face Ref) of the bulk type recording medium 100 when the biaxial actuator 118 is driven through a focus servo control by a servo circuit 125 described later. In addition, a position in the tracking direction of the servo laser light follows the guide grooves formed on the selective reflection layer 103 when the biaxial actuator 118 is driven through a tracking servo control by the servo circuit 125.
Reflection light of the servo laser light from the selective reflection layer 103 is reflected by the dichroic mirror 116 via the objective lens 117, and then is reflected by the beam splitter 121. The reflection light of the servo laser light reflected by the beam splitter 121 is collected on a detection surface of a photo detector 123 via a condensing lens 122.
A matrix circuit 124 generates each of focus and tracking error signals based on light detecting signals from the photo detector 123, and each error signal is supplied to the servo circuit 125.
The servo circuit 125 generates a focus servo signal and a tracking servo signal from each error signal. When the biaxial actuator 118 is driven based on the focus servo signal and the tracking servo signal, the focus servo control and the tracking servo control of the objective lens 117 are realized.
Here, when mark recording is performed on a necessary information recording layer position L among the respective information recording layer positions L set in the bulk type recording medium 100 in advance, a focus position of the recording laser light is varied by an amount according to an offset corresponding to a selected information recording layer position L by controlling driving of the lens driving portion 115.
Specifically, such a setting control of the information recording position is performed by, for example, a controller 126 which controls the overall recording apparatus. That is to say, the controller 126 controls the lens driving portion 115 based on an offset amount of-L which is set in advance according to a targeted information recording layer position Ln, and an information recording position (focus position) by the recording laser light focuses on the targeted information recording layer position Ln.
In addition, the tracking servo of the recording laser light during recording, as described above, is automatically performed when the servo circuit 125 performs the tracking servo control for the objective lens 117 based on reflection light of the servo laser light. Specifically, a spot position of the recording laser light in the tracking direction is controlled to be located under the guide grooves formed on the reference face Ref.
When the bulk type recording medium 100 on which marks have been recorded is reproduced, it is not necessary to control a position of the objective lens 117 based on reflection light of the servo laser light from the reference face Ref unlike the recording. In other words, during the reproduction, it is possible to perform a focus servo control and a tracking servo control of the objective lens 117 based on reflection light of the reproduction laser light by targeting a mark string formed at the information recording layer positions L to be reproduced by applying reproduction laser light.
However, in a case of employing a specification which allows a recording part and a non-recording part to be mixed in the same layer position L, in relation to the focus servo control during the reproduction, it is preferable to employ a method in which a position of the objective lens 117 is controlled (a focal position of the recording laser light is adjusted by the recording light focus mechanism) based on reflection light of the servo laser light from the reference face Ref in the same manner as the recording. This is because focus servo misalignment of recording and reproducing laser light is prevented from occurring in the non-recording part.
As described above, in the bulk recording type, the recording laser light for recording the marks and the servo laser light as position control light are applied to the bulk type recording medium 100 through the common objective lens 117 (through the synthesis on the same optical axis). In addition, a focus servo and a tracking servo can be performed for the recording laser light even if guide grooves or a reflection surface where the guide grooves are formed are not formed in the bulk layer 102, by performing a focus servo control and a tracking servo control for the objective lens 117 based on the reflection light of the servo laser light.
However, in a case of employing the above-described servo control method, in relation to the recording laser light and the servo laser light, a spot position is misaligned in an inner direction of the recording surface due to lens shift of the objective lens 117 caused by eccentricity of the bulk type recording medium 100 or so-called skew (tilt).
FIGS. 29A and 29B schematically show a spot position misalignment of the recording laser light and the servo laser light due to occurrence of skew.
In a state where there is no skew shown in FIG. 29A, the spot positions of the servo laser light and the recording laser light correspond with each other in the recording surface inner direction. In contrast, in a case where there is skew as shown in FIG. 29B, the optical axes of the servo laser light and the recording laser light are misaligned with each other, and thus the spot position misalignment Δx occurs as shown in the figure.
FIGS. 30A and 30B schematically show a spot position misalignment of the recording laser light and the servo laser light due to lens shift.
In a state where there is no lens shift shown in FIG. 30A, the objective lens is in a reference position, and thus the center of the objective lens corresponds with the optical axis c of each laser light beam incident to the objective lens. An optical system is designed such that spot positions correspond with each other in the recording surface inner directions of the respective laser light beams in a state where the objective lens is in the reference position.
In contrast, when the objective lens is shifted from the reference position so as to track disc eccentricity as shown in FIG. 30B (in this case, the objective lens is shifted in the left direction of the figure) through the tracking servo control, the spot position misalignment Δx occurs as shown in the figure.
The spot position misalignment caused by the lens shift occurs due to a difference in incident aspects of the servo laser light and the recording laser light to the objective lens. Specifically, this is because the servo laser light is incident to the objective lens so as to be substantially parallel, whereas the recording laser light is incident thereto so as not to be parallel.
A misalignment in information recording positions occurs in the bulk layer 102 according to the occurrence of the spot position misalignment of the servo laser light and the recording laser light caused by the skew or the lens shift. That is to say, as can be seen from the above description, a spot position of the recording laser light during recording is controlled by performing a tracking servo control for the objective lens based on reflection light of the servo laser light, and thus recording may not be performed at a desired position in the bulk layer 102 because of the occurrence of the above-described spot position misalignment.
At this time, there is concern that information recording positions may overlap each other between adjacent tracks depending on an amount of skew or eccentricity of the disc to be generated or settings of track pitch (formation interval of position guiders). Specifically, since the disc eccentricity or the skew may occur in a different aspect because a disc is clamped onto a spindle motor each time discs are loaded, for example, in a case where a disc is rewritten according to disc shifting, an aspect of skew or eccentricity which has occurred in previous recording is different from an aspect of skew or eccentricity occurring during the rewriting, and thereby there may occur a problem that a mark string in the pre-recording part may overlap a mark string in the rewritten part, or they may intersect each other depending on the situation.
In this state, a recording signal may not be reproduced correctly.
As one method for preventing the overlapping or intersection of the mark strings, there may be a method in which a track pitch in the reference face Ref is set to be wide.
However, if the track pitch in the reference face Ref is widened, recording capacity is naturally reduced in the bulk layer 102.
As one method for preventing reduction in recording capacity in the bulk layer 102 while preventing the overlapping or intersection of the recorded mark strings due to the above-described skew or lens shift, employing a servo control method using a so-called ATS (Adjacent Track Servo) has been reviewed.
FIG. 31 is a diagram illustrating the ATS.
The ATS has been reviewed originally as a self servo track writer (SSTW) in a hard disc drive.
As shown in FIG. 31, in the ATS, an adjacent track servo spot Sats is formed on a recording medium along with a recording spot Srec (irradiation spot of the recording laser light).
In the ATS, the recording spot Srec is a leading spot (for example, an outer circumferential side in a case where recording progresses from the inner circumference to the outer circumference in the radius direction), the adjacent track servo spot Sats is a following spot, and a tracking servo is performed on a mark string formed by the recording spot Srec using the adjacent track servo spot Sats. That is to say, a tracking servo control of the objective lens is performed such that the adjacent track servo spot Sats follows a track previous to one track of tracks formed by the recording spot Srec.
According to the ATS, since a track pitch is a distance S between spots and is constant, it is possible to effectively prevent the problem that the tracks overlap each other (information recording positions overlap each other) due to the influence of the eccentricity.
Here, when recording is started by the ATS, first, tracking servo pull-in is performed on a pre-recording part by the adjacent track servo spot Sats.
Specifically, when recording is started by the ATS, the tracking servo pull-in is performed on a pre-recording part by the adjacent track servo spot Sats. If the tracking servo pull-in is successful, address information recorded in the pre-recorded mark strings can be read by the adjacent track servo spot Sats, and thus it is possible to detect a timing when the adjacent track servo spot Sats reaches a position which is previous to one circumference of an end position of the pre-recording part. In this way, it is possible to start rewriting on a previously recording part by starting recording using the recording spot Srec at the timing when the adjacent track servo spot Sats reaches a position which is previous to one circumference of an end position of the pre-recording part.
However, it is noted that a pre-recorded mark string (recording track) is relatively moved (a so-called traverse state) with respect to the adjacent track servo spot Sats due to the influence of the disc eccentricity when the pull-in is performed.
FIG. 32 is a diagram illustrating a traverse signal (a tracking error signal obtained in a traverse state) accompanied by the disc eccentricity.
If the disc eccentricity occurs, a movement speed of a track which traverses a spot varies at a period corresponding to rotation of the disc. For this reason, the traverse signal accompanied by the disc eccentricity can be obtained by a waveform where high and low frequencies thereof are repeated periodically as shown in the figure.
Here, a movement speed of the track is preferably low when the tracking servo pull-in is performed.
In light of this point, in the related art, there has been proposed a method in which a speed is controlled such that a relative speed of the track to the spot is equal to or less than a predetermined speed and then the tracking servo pull-in is started, for example, as disclosed in Japanese Unexamined Patent Application Publication No. 02-158920.
Alternatively, more simply, a method has been generally performed in which the tracking servo pull-in is started when a relative speed of the track to the spot is equal to or less than a predetermined speed without performing a speed control.
FIG. 33 shows a configuration for realizing the latter method, that is, a pull-in method as a related example where the tracking servo pull-in is started when a relative speed of the track to the spot is equal to or less than a predetermined speed.
In the figure, a tracking servo circuit 130 generates a tracking servo signal TS for making a value of a tracking error signal TE a constant target value (for example, zero) by performing a predetermined filter process for the tracking error signal TE. A tracking driver 132 drives a tracking actuator 133 which maintains the objective lens so as to be displaced in the tracking direction, depending on a tracking drive signal TD which is generated based on the tracking servo signal TS.
With this configuration, in a state where a tracking servo loop is turned on (the filter process is in an ON state), there is performed a control for making a value of the tracking error signal TE a constant target value, that is, a tracking servo control for enabling a beam spot to follow a track.
In this case, a frequency condition determination circuit 131 is provided as a configuration for instructing the tracking servo circuit 130 to start the tracking servo pull-in.
The frequency condition determination circuit 131 measures a frequency of the tracking error signal TE (that is, the traverse signal) in a state where the tracking servo loop is turned off, and instructs the tracking servo circuit 130 to start the tracking servo pull-in when the frequency becomes equal to or less than a predetermined threshold value.
With this, when the recording tracking is moved with respect to the beam spot due to the influence of the disc eccentricity, the pull-in can be started at a timing when a relative speed is low, and thus it is possible to realize more stable tracking servo pull-in.